The invention relates to a fuel injector comprising an injector body having a cylindrical bore communicating with a fuel supply passage and containing a plunger which can move in the longitudinal direction and which is connected to a moving part of an electromagnetic drive system to be energized by a current with pulse-shaped waveform supplied by a controlled power source, and which is provided with a detector for the plunger displacement whose output signal is supplied as a feedback signal to the power-source control circuit.